[rpc] add new submitpackage RPC

It could be unsafe/confusing to create an actual mainnet interface while
package relay doesn't exist. However, a regtest-only interface allows
wallet/application devs to test current package policies.

src/test/fuzz/rpc.cpp

@@ -159,6 +159,7 @@ const std::vector<std::string> RPC_COMMANDS_SAFE_FOR_FUZZING{
     "signrawtransactionwithkey",
     "submitblock",
     "submitheader",
+    "submitpackage",
     "syncwithvalidationinterfacequeue",
     "testmempoolaccept",
     "uptime",